Medium tension circuit breakers (i.e. in the range 3 kilovolts to 45 kilovolts) as manufactured at present on the above-indicated principle cannot interrupt currents of more than 25 kiloamps when the operating energy is low (less than 300 joules). However, with the present increase in power being conveyed by medium tension lines, it is becoming necessary to have apparatus available which is capable of interrupting higher currents, e.g. as high as 45 kiloamps to 50 kiloamps.
An object of the present invention is to provide such apparatus.
Another object of the invention is to provide apparatus which is unsophisticated, and therefore relatively cheap to manufacture and to maintain.
Another object of the invention is to provide a circuit breaker that requires little operating energy, so that the cost of its control system is low.
The circuit breaker of the invention is of the type in which the increase in pressure on tripping due to an electric arc appearing between the arcing contacts is made use of to exert a driving force on the moving equipment, thereby applying additional energy to the operating member which therefore does not itself need to be too powerful. In high tension applications, such an arrangement is known, as described, for example, in French Pat. No. 85 00610, in German patent application No. 31 32 825, and in U.S. Pat. No. 2 957 063. In circuit breakers of this type, the increase in pressure which occurs in the vicinity of the arc propagates to a piston connected to the moving equipment and provides additional drive.
The ease and the speed with which pressure propagates depend firstly on the obstacles on the gas flow path between the arcing zone and the piston, and secondly on the way the pressure gradient changes between the arcing zone and the face of the piston.
In the devices described in the above-mentioned documents, the gas propagates along annular ducts whose small section does not encourage rapid flow; in addition, the pressure gradient between the arcing zone and the face of the piston falls off very quickly such that the mechanical action of gas falls off very quickly after the arc has appeared.
In order to mitigate these drawbacks, the invention proposes providing a circuit breaker in which pressure is transmitted quickly to the piston connected to the operating member and in which the flow of gas from the arcing zone takes place quickly and without disturbance.
Another object of the invention is to provide a circuit breaker in which the arc is subjected, when the circuit breaker is opened, to two blasts, one in the moving direction of the moving parts, and the other in the opposite direction.